Alignment Sensing & Stabilization system

Contents

Overview

This is a dither based system. The idea is to dither (in pitch and yaw) all of the interferometer optics. We then read out the power (arm powers, PRC power, etc.) and demodulate at the dither frequencies. We can then feedback to control the full interferometer alignment.
The concept is the same as what we use to do the initial alignment of the interferometer. It was previously tried by Valera and Bram, but never worked for some reason...
- [attachment:ass.png Conceptual Diagram]
- See also the old Alignment Sensing and Control.

The input signals to ASS are:

These are displayed on the ASS MASTER display.

Below are two diagrams which illustrate the proposed method to align the 40m IFO while it is locked. Currently the controls are done using Perl scripting.
In the diagrams the dither signals to the optics are in the YAW axis of the optics, but this is done in a similar manner in the PIT axis. The labels in the colored boxes are the channels (shown in YAW, but valid for PIT as well) to which the obtained error-signals, from the demodulation, are fed back to.

attachment:DC_ASC_Inline.png

Figure 1. Diagram of the 40m IFO in-line arm cavity (XARM). The PRM and BS are omitted for clarity.

In-line cavity legend - Figure 1:

IP_POS	Otherwise known as PZT1, located in front of the PRM. Steers the beam into the IFO.
IP_ANG	Otherwise known as PZT2, located after IP_POS and before the PRM. Also, steers the beam into the IFO.
ITMx	The ITM of the x-arm, ITMX.
ETMx	The ETM of the x-arm, ETMX.
PDH-err	The demodulated length control signal, C1:LSC-XARM_IN1.
TRX	The transmitted power of the x-arm, C1:LSC-TRX_OUT.
L.O. ITMx	Modulation of the ITMx in PIT or YAW using tdssine.
L.O. ETMx	Modulation of the ETMx in PIt or YAW using tdssine.

attachment:DC_ASC_Perp.png

Perpendicular arm legend - Figure 2:

BS	Beamsplitter, steers the beam into the perpendicular arm cavity (y-arm).
ITMy	The ITM of the y-arm, ITMY.
ETMy	The ETM of the y-arm, ETMY.
PDH-err	The demodulated length control signal, C1:LSC-YARM_IN1.
TRY	The transmitted power of the y-arm, C1:LSC-TRY_OUT.
L.O. ITMy	Modulation of the ITMy in PIT or YAW using tdssine.
L.O. ETMy	Modulation of the ETMy in PIt or YAW using tdssine.

Diagram for digital dither alignment scheme of the DRFPMI attachment:40mASC.pdf